1. Field of the Invention
The present invention relates to an image conversion apparatus and an image conversion method converting a video signal outputted from computer into a television signal.
2. Description of the Related Art
Recently, in the field of computer, it is possible for a machine such as personal computer to display a game and a CG (computer graphic) using a three dimensional display without incompatibility with rapid improvement in the processing speed of processors and the performance of graphic chips. It is also considered that the infiltration of computer into a home further advances in future since a price of personal computer has lowered by the improvement of integration of an IC.
However, since a CRT display unit connected to such computer has restrictions for manufacturing, it is difficult to obtain a good progress for making the CRT display unit thinner and larger and lowering the price thereof. Therefore, conventionally, there is a case in that computer is connected to a television receiver with a big screen for home use in order to quickly display the game and the CG image using three dimensional display on the big screen. In this case, it is necessary to put an image conversion apparatus converting a video signal outputted from computer (analog RGB signal) into a television signal (composite NTSC signal, PAL signal, separate Y/C signal) between a RGB output terminal of the computer and a video input terminal of the television receiver.
The image conversion apparatus, in order to execute this signal conversion, must execute various operations in addition to that the RGB signal is converted into a YUV signal (further, a YIQ signal) and the scanning method of the video signal is converted from a non-interlace scanning into an interlace scanning.
That is the CRT display unit and the television receiver are different not only in scanning frequencies but also in scanning, namely, the CRT display unit executes under-scanning (an image is displayed in a narrower range than a screen) while the television device executes over-scanning (an image is displayed in a larger range than a screen). Accordingly, the image conversion apparatus must apply an operation converting the scanning frequency and a scaling operation reducing and magnifying an image to the video signal which is originally outputted for the CRT display unit. Further, when the scanning mode of the video signal is converted from the non-interlace scanning into the interlace scanning, components of 1/2 frequency components of vertical frequency produce, therefore, a flicker in which points of the high frequency appear to be blinking produces in the screen of the television receiver. Accordingly, in order to prevent an occurrence of such flicker, the image conversion apparatus must previously apply an smooth-filtering operation reducing high frequency components which will turn back to the video signal before converting the scanning mode of the video signal from the non-interlace scanning method into the interlace scanning mode.
FIG. 5 is a block diagram showing a configuration of a conventional image conversion apparatus with an arithmetic circuit executing this operation. In FIG. 5, the image conversion apparatus is provided with an A/D (analog/digital) converter 50, a RGB matrix converting circuit 51, a line memory 52, an arithmetic circuit 53, and a video encoder 54 connected sequentially from a computer side not shown (left side in FIG. 5), and a timing generation circuit 55 connected to all of them.
The A/D converter 50 is a circuit concerting each of RGB analog video signals outputted from the computer not shown into a digital signal. Incidentally, the A/D converter 50 is omitted when a digital signal is directly outputted from the computer not shown.
The RGB matrix converting circuit 51 is a circuit obtaining a YUV signal or a YIQ signal by applying, every pixel of each color digital video signal received from the A/D converter 50, a matrix operation according to a predetermined operation coefficient to each RGB luminance value in the pixel.
The line memory circuit 52 is a memory storing data (YIQ signal) of several lines (here, "1 line" indicating video signals for 1 horizontal synchronization period), and functions as a delay line.
The arithmetic circuit 53 reads data (YIQ signal) of several lines from the line memory circuit 52 at a time, and applies the scaling operation and the filtering operation stated above to the data.
The video encoder 54 converts the scanning mode into the interlace scanning for the data (YIQ signal) processed by the arithmetic circuit 53 by changing the order of each line, and converts the video signal into a television signal such as the composite NTSC signal or the PAL signal by executing a predetermined synthesis process. Then, the video encoder 54 inputs the television signal into a video input terminal of the television receiver not shown so as to display the image processed by the computer not shown on a television screen.
The timing generation circuit 55 generates predetermined timing signals for the video signal (such as a horizontal synchronous signal, a vertical synchronous signal, and a color modulation reference signal), and controls the line memory circuit 52.
However, the arithmetic circuit 53 in the conventional image conversion apparatus executes the various arithmetic processes as a simple linear operation without taking the luminous property (.gamma. property) of the CRT into consideration. Therefore, the conventional image conversion apparatus produces the following problems latently.
In other words, the most popular CRT as a display unit of the television receiver has a nonlinear luminous property (.gamma. property) shown in the follows expression (1), therefore, it is impossible to make the luminosity be in proportion to the level of the input signal. EQU a=Kb.gamma. (1)
where "a" is luminosity, "k" is a proportional constant which is peculiar every apparatus, and "b" is an input voltage. Therefore, the correction (.gamma. correction) shown in the follows expression (2) is previously applied to a broadcast wave in order to keep an inverse .gamma. property counteracting the luminous property (.gamma. property) of the CRT. EQU C'=c.sup.1/.gamma. (2)
where "c" is a voltage which is virtually in proportion to the luminance of each color R, G or B on the CRT as to the analog RGB signal outputted from the computer and is a voltage in proportion to the original luminance of the image on the CRT as to the television signal. Further, c' is a voltage of each RGB color signal actually outputted from the computer as to the analog RGB signal, and is a voltage of RGB signal before the color modulation as to the television signal. As the value of ".gamma.", 2.2 is set as a reference value for the NTSC, and 2.8 is set as a reference value for the PAL.
The video signal which is virtually .gamma. corrected is inputted into the arithmetic circuit 53 in the conventional image conversion apparatus. However, the arithmetic circuit 53 in the conventional image conversion apparatus applies only the linear operation to the video signal which is provided with the nonlinear property by the .gamma. correction. Accordingly, the variation ratio of the signal is not mutually consistent with the variation ratio of the luminance in the image by the linear operation on the display unit of the television receiver.
For example, as shown FIGS. 3(a) and 3(b), it is assumed that a smooth-filtering operation of which the impulse response is (1/4, 1/2, 1/4) to the luminance signal Y in the arithmetic circuit 53. In this case, when the .gamma.-corrected luminance signal Y is inputted into the arithmetic circuit 53 as shown in FIG. 6(a), the arithmetic circuit 53 sets a luminance value of a pixel in a line to be processed to 1/2.multidot.Y (=0.5) and adds 1/4.multidot.Y (=0.25) to each of luminance values of pixels vertically adjacent to each other. When such the filtered luminance signal Y is inputted into the television receiver, the luminance on the display unit is shown by the above described expression (1) because of .gamma. property of the television receiver. That is, in a case of .gamma.=2.8 (PAL), as to the luminance signal Y, EQU A=0.5.sup.2.8 =0.14 (where the proportional constant K=1) (3)
and each additional luminance to each of the vertical pixels, EQU A=0.25.sup.2.8 =0.02 (where the proportional constant k=1) (4).
As above described, on the display unit of the television receiver, the luminance values of the pixels in the line to be processed and the luminance values of the pixels vertically adjacent to each other is not consistent with the ratio of (1/4, 1/2, 1/4) (refer to FIG. 6(b)), therefore, it results in that the response of the signal is distorted.